Cryogenic refrigerator

ABSTRACT

A cryogenic refrigerator includes : a axially extending cylinder; an axially reciprocating displacer provided inside the cylinder, at a gap between an inner circumferential surface of the cylinder and an outer circumferential surface of the displacer, the displacer shifting to create an expansion space between the displacer and a first axial end portion of the cylinder; a regenerator built in the displacer; and a sleeve disposed along the inner circumferential surface of the first axial end portion of the cylinder, encompassing the expansion space. A first passage for guiding the refrigerant gas from the regenerator to the gap is provided in the displacer, and a second passage for guiding the refrigerant gas from the gap to the expansion space is provided between the first axial end of the cylinder and the sleeve, and/or is provided between the outer surface and the inner surface of the sleeve.

RELATED APPLICATIONS

Priority is claimed to Japanese Patent Application No. 2014-206156,filed Oct. 7, 2014, the entire content of which is incorporated hereinby reference.

BACKGROUND

1. Technical Field

Certain embodiments of the invention relate to a cryogenic refrigerator.

2.Description of Related Art

Cryogenic refrigerators are used to cool a refrigeration article down totemperatures in a range of, for example, from about 100 K (Kelvin) toabout 4 K. Examples of cryogenic refrigerators include Gifford-McMahon(GM) refrigerators, pulse tube refrigerators, Stirling refrigerators,and the Solvay refrigerator. Cryogenic refrigerators are used, forexample, for cooling superconducting magnets or detectors, or incryopumps.

SUMMARY

According to a certain embodiment of the invention, there is provided acryogenic refrigerator including: an axially extending cylinder; anaxially reciprocating displacer provided inside the cylinder, at a gapbetween an inner circumferential surface of the cylinder and an outercircumferential surface of the displacer, for shifting to create anexpansion space for refrigerant gas between the displacer and an axialend portion of the cylinder; a regenerator built into the displacer; anda sleeve disposed along the inner circumferential surface of the axialend portion of the cylinder, encompassing the expansion space. A firstpassage for guiding the refrigerant gas from the regenerator to the gapis provided in the displacer, and a second passage for guiding therefrigerant gas from the gap to the expansion space is provided betweenthe axial end portion of the cylinder and the sleeve, and/or is providedbetween the outer surface and the inner surface of the sleeve.

According to embodiments of the invention, it is possible to enhanceheat exchanging efficiency of a cryogenic refrigerator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a cryogenic refrigeratoraccording to an embodiment of the invention.

FIG. 2 is a schematic top view of a sleeve according to an embodiment ofthe invention.

FIG. 3 is a schematic top view of a sleeve according to anotherembodiment of the invention.

FIG. 4 is a schematic top view of a sleeve according to still anotherembodiment of the invention.

DETAILED DESCRIPTION

The need for improving heat exchanging efficiency has been felt in theart of cryogenic refrigerators.

Hereinafter, embodiments of the invention will be described in detailwith reference to the drawings. In addition, the same reference signsare assigned to the same components in the description, and theduplicated description is appropriately omitted. The configurationdescribed below is an example, and does not limit the scope of theinvention.

FIG. 1 is a schematic diagram illustrating a cryogenic refrigeratoraccording to an embodiment of the invention. The cryogenic refrigeratoris, for example, a GM refrigerator 10. The illustrated GM refrigerator10 is a single-stage refrigerator. The GM refrigerator 10 uses a heliumgas, for example, as a refrigerant gas.

The regenerator-type cryogenic refrigerator such as the GM refrigerator10 includes a regenerator 12, an expander 14, and a compressor 16. Asillustrated in FIG. 1, the regenerator 12 is provided in the expander14, and is configured to pre-cool the high-pressure refrigerant gas thatis supplied from the compressor 16 to the expander 14. The expander 14includes an expansion space 18 of the refrigerant gas. The refrigerantgas that is pre-cooled by the regenerator 12 is expanded in theexpansion space 18 and is further cooled. The regenerator 12 isconfigured to be cooled by the refrigerant gas that is cooled by theexpansion. The compressor 16 is configured to collect the refrigerantgas from the regenerator 12, to compress the refrigerant gas, and tosupply the refrigerant gas to the regenerator 12 and the expansion space18 again.

The expander 14 includes a cold head including a cylinder 20, a coolingstage 22, and a displacer 24. The cylinder 20 is an air-tight containerof the refrigerant gas, and is a hollow member that extends in an axialdirection Q. The cylinder 20 has, for example, a cylindrical shape.

The cooling stage 22 is thermally connected to the cylinder 20 bysurrounding the expansion space 18. The cooling stage 22 is formed tohave, for example, a bottomed cylindrical shape, and is attached to theouter side of the cylinder 20. The cooling stage 22 functions as a heatexchanger that performs heat exchange between the refrigerant gas and acooling object such as an external heat source. The cooling stage 22 maybe called a thermal load flange.

The displacer 24 is arranged on the same axis as the cylinder 20. Theregenerator 12 is built in the displacer 24. The displacer 24 has, forexample, a cylindrical shape having a diameter that is slightly smallerthan that of the cylinder 20. A gap is provided between the innercircumferential surface of the cylinder 20 and the outer circumferentialsurface of the displacer 24. This gap is referred to as a firstclearance 26 below. The outer circumferential surface of the displacer24 is a side surface of the displacer 24, and the inner circumferentialsurface of the cylinder 20 is the surface of the cylinder 20 facing theside surface of the displacer 24.

The displacer 24 is a piston that divides the internal space of thecylinder 20 into the expansion space 18 and a room temperature space 28.The expansion space 18 is formed on one side of the cylinder 20 withrespect to the displacer 24, and the room temperature space 28 is formedon the other side of the cylinder 20 with respect to the displacer 24.Therefore, one end portion of the cylinder 20 (or the displacer 24) inthe axial direction Q can be called a low temperature end, and the otherend of the cylinder 20 (or the displacer 24) in the axial direction Qcan be called a high temperature end. Accordingly, the expansion space18 is formed between the low temperature end of the displacer 24 and thelow temperature end of the cylinder 20, and the room temperature space28 is formed between the high temperature end of the displacer 24 andthe high temperature end of the cylinder 20.

Hereinafter, for the convenience of description, the relative positionalrelationship between elements may be described by representing the roomtemperature side as “upper” and the low temperature side as “lower”. Forexample, it is possible to describe that the room temperature space 28is present at the upper portion of the displacer 24 and the expansionspace 18 is present at the lower portion of the displacer 24.

The displacer 24 is provided in the cylinder 20 so as to move in theaxial direction Q in a reciprocating manner. A driving unit 25 isconnected to the high temperature end of the displacer 24 for thereciprocating movement of the displacer 24. By the reciprocatingmovement of the displacer 24, the volumes of the expansion space 18 andthe room temperature space 28 are complementarily changed.

A displacer upper opening 30 is provided to the high temperature end ofthe displacer 24 in order to cause the refrigerant gas to flow betweenthe room temperature space 28 and the regenerator 12. The displacerupper opening 30 is formed along the axial direction Q. A displacerlower opening 32 is provided to the low temperature end of the displacer24 in order to cause the refrigerant gas to flow between the regenerator12 and the expansion space 18. The displacer lower opening 32 is apassage that guides the refrigerant gas from the low temperature end ofthe regenerator 12 to the first clearance 26. The displacer loweropening 32 is formed along a radial direction that is orthogonal to theaxial direction Q.

A seal 34 may be provided at the upper portion of the first clearance26. The flow of the gas that has passed through the first clearance 26is blocked by the seal 34. Accordingly, the flow of the refrigerant gasbetween the room temperature space 28 and the expansion space 18 passesthrough the regenerator 12. In a case where the seal 34 is a contactseal such as a seal ring, the seal 34 may be provided to the hightemperature end of the displacer 24. The seal 34 may be a non-contactseal. In addition, in a certain embodiment, the flow or leaking of therefrigerant gas that has passed through the first clearance 26 may beallowed.

In addition, the expander 14 includes a sleeve 36 that is arrangedaround the expansion space 18 at the inside of the low temperature endof the cylinder 20. The sleeve 36 is arranged on the same axis as thecylinder 20. The sleeve 36 is mounted to the low temperature end of thecylinder 20. Therefore, at least one contacting portion (notillustrated) that is in contact with the inner surface of the cylinder20 may be provided on the outer surface of the sleeve 36. The sleeve 36may be formed of the same material (for example, stainless steel) as thecylinder 20.

The sleeve 36 defines the passage that guides the refrigerant gas fromthe first clearance 26 to the expansion space 18. This gas passage is agap formed between the low temperature end of the cylinder 20 and thesleeve 36. Hereinafter, this gap is called a second clearance 38. Thesecond clearance 38 is narrower than the first clearance 26. That is,the width of the second clearance 38 in the radial direction is smallerthan the width of the first clearance 26 in the radial direction. Thesleeve 36 configures a flow velocity increasing mechanism for therefrigerant gas in the cooling stage 22.

FIG. 2 is a schematic top view of the sleeve 36 according to a certainembodiment of the invention. As illustrated in FIGS. 1 and 2, the sleeve36 includes a sleeve cylindrical portion 40 that faces the innercircumferential surface of the cylinder 20, and a sleeve bottom plate 42that faces the bottom portion of the cylinder 20. The sleeve cylindricalportion 40 extends in the axial direction Q along the innercircumferential surface of the cylinder 20 at the low temperature end ofthe cylinder 20. The sleeve bottom plate 42 extends from the sleevecylindrical portion 40 toward the inside in the radial direction. Inthis manner, the sleeve 36 is formed to have a bottomed cylindricalshape. The sleeve cylindrical portion 40 is, for example, a shortcylinder that extends in the axial direction Q, and has a diameter thatis slightly smaller than the inner diameter of the cylinder 20. Thesleeve bottom plate 42 is a disk that is attached to a lower end of thesleeve cylindrical portion 40.

As illustrated in FIG. 1, the second clearance 38 includes a lateral gap44, which is formed between the sleeve cylindrical portion 40 and theinner circumferential surface of the cylinder 20, and a bottom gap 46,which is formed between the sleeve bottom plate 42 and the bottomportion of the cylinder 20 and is connected to the lateral gap 44. Thesleeve bottom plate 42 has a through hole 48 at the center thereof, thethrough hole 48 allows the bottom gap 46 to communicate with theexpansion space 18. In this manner, the flow path of the refrigerant gascan be extended to the through hole 48.

The position of a sleeve upper end 50 in the axial direction issubstantially the same as that of a cooling stage upper end 23 in theaxial direction. Accordingly, a gas inlet from the first clearance 26 tothe second clearance 38 is provided at substantially the same height asthat of the cooling stage upper end 23. The gas inlet may be provided ata height different from the height of the cooling stage upper end 23. Inaddition, a gas outlet (that is, the through hole 48) from the secondclearance 38 to the expansion space 18 is provided at the same positionas that of a bottom center 49 of the cooling stage 22 in the radialdirection. The gas outlet may be provided at a position different fromthe position of the bottom center 49.

In this manner, the sleeve 36 forms the flow path of the refrigerant gasbetween the cooling stage 22 and the sleeve 36. This flow path reachesthe bottom center 49 of the cooling stage 22 from the cooling stageupper end 23 along the inner surface of the cylinder 20. The sleeve 36provides the flow path that causes the refrigerant gas to flow inparallel with the inner surface of the cooling stage 22 in almost theentire area of the inner surface of the cooling stage 22. In FIG. 1, theflow of the refrigerant gas in the lateral gap 44 is indicated by arrowsA, and the flow of the refrigerant gas in the bottom gap 46 is indicatedby arrows B. In addition, the flow of gas passing through the throughhole 48 is indicated by an arrow C.

In the movable range in the axial direction (hereinafter, referred to asa stroke) of the displacer 24, the displacer lower opening 32 is usuallypositioned at the upper portion of the sleeve upper end 50 in the axialdirection Q. The displacer lower opening 32 is usually positioned at theupper portion of the second clearance 38 and does not enter the insideof the sleeve 36. Therefore, the displacer lower opening 32 is nothidden by the sleeve 36 from the cylinder 20 (or the cooling stage 22).In addition, in a certain embodiment, in at least a portion of thestroke (for example, when the displacer 24 is at the bottom deadcenter), the displacer lower opening 32 may be positioned at the lowerportion of the sleeve upper end 50 in the axial direction Q.

The sleeve upper end 50 defines an opening that receives the lowtemperature end of the displacer 24. In the stroke of the displacer 24,the low temperature end of the displacer 24 is usually inserted into thesleeve 36. In other words, the movable range of a displacer bottomsurface 33 is in the sleeve 36. The sleeve upper end 50 is inserted intothe lower portion of the first clearance 26, and the sleeve cylindricalportion 40 surrounds the low temperature end of the displacer 24. Inaddition, in a certain embodiment, in at least a portion of the stroke(for example, when the displacer 24 is at the top dead center) or entirestroke, the displacer bottom surface 33 may be at the outside of thesleeve 36.

A gap in the radial direction, which is formed between the sleevecylindrical portion 40 and the low temperature end of the displacer 24when the displacer 24 is inserted into the sleeve 36, is narrower thanthe lateral gap 44. That is, the width of the gap in the radialdirection is smaller than the width of the lateral gap 44 in the radialdirection. In this manner, it is possible to increase the flow rate ofthe gas passing through the lateral gap 44.

The sleeve 36 may provide a seal between the low temperature end of thedisplacer 24 and the sleeve 36. The seal may be a contact seal or anon-contact seal. A direct gas flow from the first clearance 26 to theexpansion space 18 is blocked by the seal. Accordingly, all the flow ofthe refrigerant gas between the first clearance 26 and the expansionspace 18 passes through the second clearance 38. In this case, the innersurface of the sleeve cylindrical portion 40 may be in contact with theouter circumferential surface of the low temperature end of thedisplacer 24. Otherwise, the inner surface of the sleeve cylindricalportion 40 may be in non-contact with the outer circumferential surfaceof the low temperature end of the displacer 24 by providing a slight gaptherebetween. According to the reciprocating movement of the displacer24, the low temperature end of the displacer 24 moves in a slidingmanner or in a non-contact manner with respect to the sleeve 36.

In addition, the GM refrigerator 10 includes a piping system 52 thatconnects the compressor 16 to the expander 14. In the piping system 52,a high pressure valve 54 and a low pressure valve 56 are provided. Thepiping system 52 is connected to the high temperature end of thecylinder 20. The GM refrigerator 10 is configured to supply thehigh-pressure refrigerant gas from the compressor 16 to the expander 14via the high pressure valve 54 and the piping system 52. In addition,the GM refrigerator 10 is configured to discharge the low-pressurerefrigerant gas from the expander 14 to the compressor 16 via the pipingsystem 52 and the low pressure valve 56.

The GM refrigerator 10 includes a valve driving unit (not illustrated)that selectively closes and opens the high pressure valve 54 and the lowpressure valve 56 in synchronization with the reciprocating movement ofthe displacer 24, and switches between the supply and the discharge ofthe refrigerant gas with respect to the expansion space 18. The valvedriving unit may be the driving unit 25 described above. The highpressure valve 54, the low pressure valve 56, and the valve driving unitmay be incorporated in the expander 14.

Next, the operation of the GM refrigerator 10 is described. When thedisplacer 24 is positioned at the bottom dead center or in the vicinityof the bottom dead center of the cylinder 20, the high pressure valve 54is opened. The high-pressure refrigerant gas is supplied from thecompressor 16 to the cylinder 20 via the high pressure valve 54 and thepiping system 52. The refrigerant gas flows into the regenerator 12 fromthe room temperature space 28 via the displacer upper opening 30. Therefrigerant gas is cooled while passing through the regenerator 12. Therefrigerant gas flows into the expansion space 18 via the displacerlower opening 32, the first clearance 26, and the second clearance 38.While the refrigerant gas flows into the expansion space 18, thedisplacer 24 moves toward the top dead center of the cylinder 20. Inthis manner, the volume of the expansion space 18 is increased.Accordingly, the expansion space 18 is filled with the high-pressurerefrigerant gas.

When the displacer 24 is positioned at the top dead center or in thevicinity of the top dead center of the cylinder 20, the high pressurevalve 54 is closed. At the same timing as, or slightly after, the highpressure valve 54 is closed, the low pressure valve 56 is opened. Therefrigerant gas of the expansion space 18 is expanded and cooled. Therefrigerant gas absorbs the heat from the cooling stage 22.

The low-pressure refrigerant gas is collected in a reversed route. Therefrigerant gas flows into the regenerator 12 from the expansion space18 via the second clearance 38, the first clearance 26, and thedisplacer lower opening 32. The refrigerant gas cools the regenerator 12while passing through the regenerator 12. The refrigerant gas isdischarged from the cylinder 20 via the displacer upper opening 30 andthe room temperature space 28. The refrigerant gas is collected by thecompressor 16 via the low pressure valve 56 and the piping system 52.While the refrigerant gas flows out from the expansion space 18, thedisplacer 24 moves toward the bottom dead center of the cylinder 20. Inthis manner, the volume of the expansion space is decreased, and thelow-pressure refrigerant gas is discharged from the expansion space 18.

One cooling cycle in the GM refrigerator 10 is described above. The GMrefrigerator 10 repeatedly performs this cooling cycle, and therefore,the cooling stage 22 is cooled to a desired temperature. In this manner,the GM refrigerator 10 can absorb the heat from the cooling object (notillustrated) that is thermally connected to the cooling stage 22 and cancool the cooling object. The cooling stage 22 may be cooled to a targettemperature selected from a range of, for example, about 10 K to about30 K. Otherwise, the cooling stage 22 may be cooled to a targettemperature selected from a range of, for example, about 50 K to 100 K.

As described above, according to the embodiment, the passage of therefrigerant gas from the first clearance 26 to the expansion space 18(that is, the second clearance 38) is defined by providing the sleeve 36to the inside of the cylinder 20 to be adjacent to the cooling stage 22.By defining the gas passage in this manner, the lowering of the velocitycomponent in a direction along the surface of the cooling stage 22 issuppressed compared to a case in the related art in which the gas isdirectly blown from the low temperature end of the displacer 24 to theexpansion space 18. Since the velocity can be increased compared to thatin the related art, it is possible to enhance the heat exchangingefficiency of the cooling stage 22.

The second clearance 38 is narrower than the first clearance 26.Specifically, the gas passage defined in an outside region of the sleeve36 by the sleeve 36 is narrower than the gap between the cylinder 20 andthe displacer 24 in the radial direction. Accordingly, when the gasflows in the gas passage from the gap, the velocity is increased, andtherefore, it is possible to enhance the heat exchanging efficiency.According to a trial calculation, if the velocity of the refrigerant gasflowing in the expansion space 18 is doubled, the refrigerating capacityof the refrigerator is improved by about 5% to about 10%. Therefore, asthe refrigerator is a large-sized refrigerator having a highrefrigerating capacity, the increasing amount of the refrigeratingcapacity by the application of the sleeve 36 according to the embodimentbecomes large. Typically, such a large-sized refrigerator is asingle-stage refrigerator. Accordingly, the embodiment is preferable fora single-stage refrigerator having a high capacity (for example, asingle-stage refrigerator having a refrigerating capacity of 100 W to300 W at 10 K, or a single-stage refrigerator having a refrigeratingcapacity of 500 W to 1 kW at 70 K) .

In addition, according to the embodiment, it is possible to enhance theheat exchanging efficiency of the refrigerator by a relatively simpleoperation such as mounting of the sleeve 36 to the cylinder 20. Byadding the sleeve 36 to the existing refrigerator, it is possible toeasily enhance the heat exchanging efficiency of the refrigerator.

Herein before, the invention is described based on the embodiments. Theinvention is not limited to the embodiments described above. Thoseskilled in the art can understand that various changes in design andvarious modification examples are possible, and such modificationexamples are in the scope of the invention.

It is not essential that the sleeve 36 includes the sleeve bottom plate42. In a certain embodiment, the sleeve 36 includes only the sleevecylindrical portion 40. It can be said that the diameter of the throughhole 48 at a sleeve lower end is equal to the diameter of the sleevecylindrical portion 40.

FIG. 3 is a schematic top view of a sleeve 136 according to anotherembodiment of the invention. As illustrated in the drawing, theunevenness may be formed on the outer surface of the sleeve 136 (forexample, the sleeve cylindrical portion). In this case, a convex portion142 may be in contact with the inner surface of the cylinder 20 (or thecooling stage 22), and a refrigerant gas passage 146 may be formedbetween a concave portion 144 and the inner surface of the cylinder 20.The refrigerant gas passage 146 may be provided along the axialdirection of the cylinder 20. The inner surface of the cylinder 20 isillustrated by a broken line.

Similarly, the unevenness may be formed on the bottom surface of thesleeve bottom plate. In this case, a gas passage formed between thesleeve bottom plate and the cylinder may be provided along the radialdirection.

As an alternative, the unevenness may be formed on the inner surface ofthe cylinder. In this case, a convex portion may be in contact with theouter surface of the sleeve, and a passage of the refrigerant gas may beformed between a concave portion and the outer surface of the sleeve.

FIG. 4 is a schematic top view of a sleeve 236 according to stillanother embodiment of the invention. The sleeve 236 (for example, sleevecylindrical portion) may define a gas passage between an outer surface238 and an inner surface 240 of the sleeve 236. This gas passage may bea through hole 242 formed in the sleeve 236. The through hole 242 may beprovided along the axial direction of the cylinder. Such a through holemay be provided to a sleeve bottom plate, and in this case, the throughhole may provided along the radial direction.

In a certain embodiment, the gas passage defined between the cylinderand the sleeve (for example, the gas passage illustrated in FIG. 1 or 3)may be used in combination with the gas passage defined between theouter surface and the inner surface of the sleeve (for example, the gaspassage illustrated in FIG. 4). For example, a gas passage is definedbetween the sleeve cylindrical portion and the cylinder, a gas passageconnected to the gas passage may be formed on the sleeve bottom plate asa through hole. Otherwise, a through hole is formed on a sleevecylindrical portion, and a gas passage connected to the through hole maybe defined between the sleeve bottom plate and the cylinder.

In a certain embodiment, in a case where the sleeve is necessarilyaccommodated, the outer diameter of the low temperature end of thedisplacer may be slightly smaller than that of the high temperature end.Otherwise, the inner diameter of the low temperature end of the cylinderor the inner diameter of the cooling stage may be slightly greater thanthat of the high temperature end of the cylinder.

In a certain embodiment, the sleeve may be provided at the lowtemperature end of at least a stage in a two-stage (or multiple-stage)refrigerator.

In the above embodiment, the GM refrigerator 10 is described as anexample, but it is not limited thereto. In a certain embodiment, asleeve may be provided in another type of refrigerator that includes adisplacer in which a regenerator is built, and a cylinder thataccommodates the displacer.

The GM refrigerator 10 or another refrigerator including the sleeveaccording to the embodiment may be used as cooling means or liquefyingmeans in a superconducting magnet, a cryopump, an X-ray detector, aninfrared sensor, a quantum photon detector, a solid state detector, adilution refrigerator, an He3 refrigerator, an insulated demagnetizedrefrigerator, a helium liquefier, and a cryostat.

It should be understood that the invention is not limited to theabove-described embodiment, but may be modified into various forms onthe basis of the spirit of the invention. Additionally, themodifications are included in the scope of the invention.

What is claimed is:
 1. A cryogenic refrigerator comprising: an axiallyextending cylinder; an axially reciprocating displacer provided insidethe cylinder, at a gap between an inner circumferential surface of thecylinder and an outer circumferential surface of the displacer, forshifting to create an expansion space for refrigerant gas between thedisplacer and an axial end portion of the cylinder; a regenerator builtinto the displacer; and a sleeve disposed along the innercircumferential surface of the axial end portion of the cylinder,encompassing the expansion space; wherein a first passage for guidingthe refrigerant gas from the regenerator to the gap is provided in thedisplacer, and a second passage for guiding the refrigerant gas from thegap to the expansion space is provided between the axial end portion ofthe cylinder and the sleeve, and/or is provided between the outersurface and the inner surface of the sleeve.
 2. The cryogenicrefrigerator according to claim 1, wherein: the sleeve includes acylindrical portion opposing the inner circumferential surface of thecylinder, and a bottom plate opposing a bottom portion of the cylinder;and the second passage includes a lateral gap formed between thecylindrical portion of the sleeve and the inner circumferential surfaceof the cylinder, and a bottom gap, connecting with the lateral gap,formed between the bottom plate of the sleeve and the bottom portion ofthe cylinder.
 3. The cryogenic refrigerator according to claim 2,wherein the bottom plate of the sleeve is centrally perforated by athrough-hole communicating the bottom gap with the expansion space. 4.The cryogenic refrigerator according to claim 1, wherein the cryogenicrefrigerator is a single-stage refrigerator.